The present invention relates to a telecommunications system especially intended for safely and reliably signaling ships in emergency condition. More particularly, the invention concerns a method for producing, detecting and interpreting in a reliable way a distress signal comprising inter alia such information as the identity and the location of a ship in emergency condition, and an apparatus for carrying out said method.
At the present time, it is common to use radioelectric (HF or VHF) telecommunications systems for signaling emergency condition at sea. Such communication links, however, are subject to wave propagation instabilities which seriously diminish the speed at which the links are established and the efficiency of such communications, which makes it uncertain whether the emergency help will be provided in time.
Improvement in safety and distress service may already be expected by using a maritime satellite which permits to improve the speed of telephone or telex communications with shore terminals. However, the telecommunication equipments which are now used are cumbersome and expensive, so that their use, under normal conditions, is currently limited to ships of relatively high burden. In case of a shipwreck, these equipments are not protected from injury due to the emergency condition encountered by the ship (e.g. explosion, fire), which renders the situation of the shipwrecked men dramatic or even catastrophic.
With such a situation there is still a need for a telecommunications system especially intended for servicing distress messages at sea, which proves to be really efficient and reliable even in the most unexpected situations and which only requires a simple ship terminal equipment that is least bulky and least expensive than a conventional telecommunications terminal.
In order to meet with the safety requirements for the distress communications, it is first preferable to provide mobile stations which are autonomous, that is stations which are not incorporated in the installations aboard the ships and which use simple telecommunications equipment. Further, the transmission should be organized to ensure high safety and efficiency, and the receive equipment at the shore terminals should be capable of responding to distress signals of very low levels and severely distorted by parasitic disturbance, thereby to enable the establishment of telecommunication links between mobile stations and shore terminals with possible relay by a maritime communications satellite.
The invention solves the above problem by providing a telecommunication process and a receive apparatus which permit to meet with the safety and reliability requirements as stated above.
A first object of this invention is a telecommunication process especially designed for producing and transmitting distress signals from simple mobile stations and for detecting and interpreting said signals in an accurate and reliable manner at shore monitoring terminals.
Another object of the invention is a receive apparatus for distress signals which is especially arranged for detecting and processing severely distorted, very low level distress signals with a high probability and accuracy, even under the most unexpected emergency conditions.
According to the invention, the telecommunication process is arranged to transmit distress signals repeatedly in succeeding sequences each comprising two parts of predetermined duration: a detect interval during which is transmitted an alarm signal, and a message transmit interval during which there is transmitted the distress message itself including inter alia information relating to the identity of the ship in emergency condition, the location thereof and the nature of emergency. During the detect interval, the transmitted signal is comprised of a carrier phase-modulated by a sub-carrier having a frequency which is a multiple of the information transmission rate. During the message transmit interval, the information is superimposed to the sub-carrier by binary phase modulation. The presence of an alarm signal during each detect interval is detected in order to produce a command signal which indicates the occurrence of a distress signal. In response to said command signal, the message signal received during the subsequent message transmit interval is coherently demodulated and processed in order to reliably restore the distress message.
The receive apparatus comprises an alarm receiver arranged to monitor the receive frequency band simultaneously in several channels having each a distinct narrow bandwidth and to produce a detect signal in response to an alarm signal being detected in any of said channels during the alarm detect interval, a transfer control logic arranged to produce a command signal in response to a detect signal being generated, and a message receiver arranged to be responsive to said command signal for processing the distress signal received during the message transmit interval in order to restore the message by coherent phase demodulation.
The message receiver comprises a controller arranged to accurately determine the carrier frequency of the received signal and to control the amplitude quality thereof in order to compensate for the frequency and phase instabilities, and thereafter to enable the reproduction of the message in conventional phase demodulator circuitries which produce a train of bits representing the distress message. The message receiver further comprises a correlator for synchronizing the message and an integrator for integrating the messages received during succeeding message transmit intervals. After having been perfectly processed so as to remove all the ambiguities resulting from the demodulation procedure, the distress message is decoded in a conventional character decoder.
Owing to their performances and the high accuracy provided in the detection and processing operation, the process and apparatus of the invention can be used with a maritime communications satellite, e.g. a MARECS type satellite, in order to relay the distress signals transmitted from the mobile stations.